It depends on oil quality/thickness and driving habits a lot. Got the car new, used a quality synthetic oil, no 0w30, no driving in 7k RPM range -> longer cylinder head's life.
Compression test on a hot engine will show whether it started leaking through the valves. Of course, removing spark plugs on a hot engine is not good. I'd be just looking at mileage, since 200+ thousand miles on that engine w/o valve lash adjustment is a good, statistically proven sign that it will burn a valve soon.